The present invention relates to a frequency hopping radio system and, more particularly, to a method for avoiding interference by co-located frequency hopping radio systems.
Spread spectrum is a communication technique originally developed to reduce eavesdropping and jamming of military radio communications. This resistance to eavesdropping and interference, both intentional and incidental, makes spread spectrum desirable for civilian uses, particularly when the communication channel is potentially crowded by a number of transmitters. One spread spectrum technique is frequency hopping where the transmission frequency is repeatedly changed during a transmission. Frequency hopping systems transmit over a substantial number of frequencies that are arranged into a relatively large number of frequency sequences or hopping patterns. For example, Federal Communications Commission (FCC) regulations in the United States (47 C.F.R. xc2xa715.247) require frequency hopping systems operating in the 2400-2483.5 MHZ band to use at least 75 different frequencies or channels with an average time of occupancy at any frequency (hop duration) not greater than 0.4 second within a 30 second period. Similar regulations constrain frequency hopping systems operating in other frequency bands and in other countries and geographical areas. The sequence of transmission frequencies used by a frequency hopping system is xe2x80x9cpseudo randomxe2x80x9d in that it appears random to receivers other than an intended receiver which has knowledge of the hopping pattern and uses that knowledge to properly tune to each frequency used in the transmission. It is this property that produces the eavesdropping and interference resistance characteristics of a frequency hopping system.
Communications in the 2.4 GHz band (also known as the Instrument, Scientific, and Medical (ISM) band) is of particular interest because this frequency band has been designated for use by unlicenced, low power transmitters. A number of short range, frequency hopping, wireless communications systems have been designed that utilize the ISM band. One such communication system is known as the xe2x80x9cHomeRF(trademark)xe2x80x9d system. The HomeRF system is intended to allow personal computers, computer peripherals, cordless telephones, and almost all other consumer electronic devices to be designed to communicate and interact with each other over a HomeRF network. The HomeRF system will carry both voice and data communications within a typical residence and will connect to the public telephone system and the Internet for longer distance communication. The HomeRF system contemplates localization or design of particular systems to conform to local telecommunication regulations. For example, in the U.S. the HomeRF system hops 50 times per second on 75 channels with 1 MHZ separation between channels. The HomeRF system utilizes 75 hopping patterns each incorporating a unique sequence of the 75 transmission frequencies. Another wireless communication system for local area use is the Bluetooth system. Like the Home RF system, the Bluetooth system is intended to provide wireless networking of virtually all digital electronic devices. The Bluetooth system utilizes 79 different transmission frequencies and hops as many as 1600 hops per second.
While frequency hopping reduces the likelihood of interference by co-located systems, collisions between co-located radio systems and other emitters (such as microwave ovens) are inevitable. The HomeRF and Bluetooth systems are designed to tolerate collisions. Both systems incorporate error detection and provide for retransmission of corrupted data. With error detection and retransmission, the primary effect of interference is a reduction in data throughput. The magnitude of the throughput reduction is proportional to the total utilization of the communication channel and the relative hopping speeds of the interfering systems. Collisions between a co-located Bluetooth system and a HomeRF system can reduce the data throughput of the HomeRF system by as much as 20%.
The HomeRF and Bluetooth systems and other similar systems operating in the ISM band are generally incompatible and do not have the capability to detect or actively avoid collisions with other co-located systems. A wireless communication system could be designed to detect and decode the transmissions of potentially interfering systems. The information obtained could be used to determine the hopping pattern in use by the interfering system. Knowledge of the hopping pattern would permit the other co-located system to avoid transmitting on a particular frequency at a time when the interferer is using that frequency. However, incorporating in each communication system the capability to receive and decode transmissions of a number of other potentially interfering systems for the purpose of collision avoidance would be prohibitively complex and expensive.
What is desired therefore, is a resource efficient method for a wireless communication system to identify potentially interfering frequency hopping radio systems and to determine the hopping pattern in use by a potentially interfering system.
The present invention overcomes the aforementioned drawbacks of the prior art by providing a method for detecting a frequency hopping pattern of a radio transmitter comprising the steps of timing an interval between detection of a signal at a predetermined frequency and detection of a signal at another predetermined frequency, and correlating that interval to a frequency hopping pattern. The frequency hopping pattern in use by a HomeRF or similar system can be identified from the time interval between the system""s transmission at different frequencies. Two time intervals separating transmission at three different frequencies are required to uniquely identify a hopping pattern from among the 75 hopping patterns used by the HomeRF system.
In addition, a method is disclosed for identifying a frequency hopping radio system comprising the steps of timing an interval between a first and a second transmission at a predetermined frequency, and correlating the interval to an identifier for the radio system. The hop sequence duration comprises the product of the duration of a hop and the number hops in a hopping pattern or sequence and is likely to be unique for each system. Timing the interval between successive transmissions at a particular frequency determines the hop sequence duration which can correlated to one of several system identifiers.
Using the methods of the invention, a system can be identified and its hop pattern determined without having knowledge of the contents of the transmissions of potentially interfering systems. Many wireless communication systems have the capability of performing these methods without additional hardware.